1. Field of the Invention
The present invention relates to a method for preparing carbon fiber fabric and, in particular, carbon fiber fabric impregnated with thermoplastic plastic.
2. Description of Related Art
Before glass fibers are formed into a fabric a protective coating is applied to the fibers. This coating is removed after the weaving by subjecting the glass fabric to a high temperature for quite a long time. A typical cycle comprises a temperature of about 400° C. for a period of 70 hours. The coating, generally designated as “strong layer,” usually consists of PVA (polyvinyl alcohol) or starch. During the heating process, the protective coating is completely burned away, which can be easily checked by determining the weight of the fabric before and after the relevant heating treatment. In order to prepare glass fabric for impregnation after the heat treatment, a more suitable coating is added to the glass fiber material for adhesion to plastic systems. Typical materials for coatings are chromium (III) and siloxane complexes.
Lightweight composite materials for structural applications are manufactured by combining carbon fibers with a plastic matrix. For space travel, aviation, and industrial applications, the plastic is added to the fibers by an impregnation process. If the impregnation process is carried out on a fiber system in which all fibers run substantially parallel to each other, the product in question is designated as unidirectional tape.
Carbon fibers are easily damaged by friction forces. However, no protection for the carbon fibers is necessary for the impregnation process for the unidirectional tape since no friction forces occur between the individual fibers.
If, however, the fibers are combined into a fabric, friction occurs between the fibers during insertion of the filling fibers perpendicular to the warp fibers. As a result of these friction forces, carbon fibers are easily damaged during the weaving process.
In order to facilitate the weaving process, carbon yarns are provided in advance by the supplier with a standard sizing, which consists of, for instance, 0.5-1% unsaturated epoxy material. This sizing does not have to be removed if the fabric is subsequently impregnated with a curing plastic system.
U.S. Pat. Nos. 4,555,446 and 4,446,255 describe how an epoxy sizing can be arranged onto carbon fibers. The sizing agent contains a solvent for arranging the agent onto the carbon fibers. Subsequently, the fibers with the sizing agent are dried at an elevated temperature in order to remove the solvent from the agent. According to U.S. Pat. No. 4,555,446, the temperature should not exceed 250° C. as the sizing would undergo heat deterioration.
The epoxy-based sizing is compatible with most thermosetting plastic systems. However, for impregnation with a thermoplastic matrix, the epoxy sizing does have an adverse effect on the adhesion between fiber and plastic.
For these reasons, material impregnated with a thermoplastic has only been made commercially available in the form of unidirectional tape. The manufacturing process of said unidirectional tapes uses fibers which are not provided with a coating or sizing. So as to enable weaving and nevertheless ensure sufficient adhesion between fibers and plastic, the epoxy sizing has to be neutralized after the weaving process.
Much attention has been devoted to this problem, and the industry is still seeking a method of removing or neutralizing the epoxy sizing on carbon fibers after the weaving process, and is seeking to find a method to improve the adhesion between carbon and thermoplastic plastic, and thus provide a basis for a reliable composite material for structural applications.